The present invention relates to temperature compensated current reference circuits. More specifically, the present invention relates to temperature compensated current reference circuits that use both a Proportional To Absolute Temperature (PTAT) current reference and an Inversely Proportional To Absolute Temperature (ITAT) current reference.
Temperature compensated current reference circuits typically employ both a PTAT current reference and an ITAT current reference. Numerous electronic circuits including current controlled oscillators, precision amplifiers and voltage regulators use temperature compensated current reference circuits in order to limit performance inaccuracies that are often caused by ambient temperature variations.
A typical PTAT current reference uses a resistor and two semiconductors of different sizes to generate a temperature dependent voltage across the resistor. The PTAT current reference has a PTAT operational amplifier, configured as a high gain differential amplifier, so that its two inputs are substantially at the same voltage. Any difference in voltage across the semiconductors, due to ambient temperature variations, is applied across the resistor and therefore the output of the PTAT operational amplifier is dependent on the current flowing through the resistor.
A typical ITAT current reference uses an ITAT operational amplifier configured as a gain differential amplifier with a semiconductor connected to one input of the differential amplifier and a resistor connected to the other input of the operational amplifier. Again, voltage variations across the ITAT configured semiconductor, due to ambient temperature variations, are applied across the resistor and therefore the output of the ITAT operational amplifier is dependent on the current flowing through the resistor.
Temperature compensated current reference circuits also typically use a current summing circuit that combines two current sources to create a combined current. One of the current sources is controlled by an output from the PTAT operational amplifier and the other one of the current sources is controlled by an output from the ITAT operational amplifier. Hence, in operation the combined current stays substantially constant, for variations in ambient temperature, since current variations in the current source controlled by the output from the PTAT operational amplifier are cancelled by current variations in the current source controlled by the output from the ITAT operational amplifier.
The above temperature compensated current reference circuits provide a relatively accurate temperature independent constant current source. However, the silicon area may be unnecessarily large, especially since the PTAT and ITAT current references each require an operational amplifier that is typically fabricated from about seven transistors plus associated biasing transistors, resistors and compensation capacitors.